1. Technical Field
The present invention relates to a technique for correcting luminance of a plurality of light emitting devices.
2. Related Art
An image forming apparatus such as a printer includes an optical head used to form an electrostatic latent image on an image carrier such as a photoconductive drum. The optical head includes a plurality of light emitting devices arranged in the form of an array in a main scan direction. Light emitting diodes may be used as the light emitting devices.
In such an optical head, a plurality of semiconductor chips having light emitting diodes formed thereon, and a separated driving IC are mounted on a print substrate with patterned wire thereon. After manufacture, there exists nonuniformity of light emission luminance between different semiconductor chips or between individual light emitting diodes in the same semiconductor chip. Such nonuniformity of light emission luminance is not preferable for print quality since it may cause unevenness of vertical stripe shapes during printing, for example. Accordingly, driving conditions from the driving IC are finely adjusted depending on characteristics of the light emitting diodes.
As correcting methods used for the optical head in the related art, the following methods are known. JP-A-6-297769 discloses a method of finely adjusting and correcting a period during which light emission current is applied. In addition, JP-A-8-39862 discloses a method of correcting a light emission driving current value using DAC. In the method disclosed in JP-A-8-39862, a plurality of correction memory circuits are provided using micromachining of a semiconductor process and a current value is finely adjusted depending on contents of a memory.
Although the method disclosed in JP-A-6-297769 is based on the principle of adjusting an area of an electrostatic latent image formed on a photoconductor drum by finely adjusting a light emission period, it is necessary to operate a circuit with very high clock frequency to make such fine adjustment of the light emission period. However, it is difficult to apply this method to an optical head integrated with a driving circuit. This is because the driving circuit is formed on a substrate even larger than conventional driving ICs and thus time constant becomes large due to parasitic capacitance, thereby making it impossible to achieve high-speed operation.
In addition, in the method disclosed in JP-A-8-39862, the machining precision of the process of forming a circuit on a large glass substrate is low compared with that of a semiconductor process on a silicon wafer, and thus it is difficult to mount equivalent correction memory circuits on the driving circuit-integrated optical head.